"A PROCESS FOR MANUFACTURE OF FLEXIBLE TERMINAL CONNECTIONS FOR USE IN LARGE TURBOGENERATORS"

Abstract

A process for manufacture of flexible terminal connections comprising Helium protected Arc Welding (He PAW) of G-Cu-CrF35 casting (5) at either end of Electrolytic Tough Pitch (ETP) grade copper flexible strips (6) having copper content of 99.9% and phosphorus content of 0.003%, using S-Cu-Sn filler metal electrode of diameter 3mm with flux and using inert gas shielding of pure helium at the flow rate of 10-151/min, and travel speed of 60-90mm/min wherein the said G-Cu-CrF35 casting (5), has copper content >97% and chromium content in the range of 0.40 to 1.4%, the said ETP grade copper flexible strips (6) are preheated to a temperature of 450 to 500°C. Fig. 3

Full Text

FIELD OF INVENTION
This invention relates to a process of manufacture of Flexible Terminal Connections for use in large turbogenerators.
PRIOR ART
Flexible Terminal Connections (FTC) (2) is a very important component of large turbogenerators and consists of a G-Cu-CrF35 casting at either end joined together with Electrolyti Tough Pitch (ETP) grade copper flexibles called expansion strips. These strips provide flexible link between bus bars (1) and terminal bushings (3) in large turbogenerators. They continuously carry very high current of the order of 20kA or more; to the terminal vibrations as well. They are designed for sudden short circuit conditions i.e. to carry current of the order of around 15 times the rated value for a short duration accompanied by heavy vibrational jerks.
According to one of the process known in the art, Flexible Terminal Connections are manufactured by welding at either end to G-Cu-Cr F35 precision sand castings using Electronic Beam Welding (EBW) (4) process.
A drawback of the above process is that the EBW process is expensive and requires expensive infrastructure.

Another drawback of the above process is that EBW technology is not available in many countries including India for Welding of copper components.
Still another drawback of the above process is that EBW technology requires to be carried out in vacuum of 10~2.Torr.
Further drawback of the above process is that components to be welded require highly precise machining at the point of welding.
Still further drawback of the EBW technology is that it requires highly experienced skill operators.
OBJECTS OF PRESENT INVENTION
An object of the present invention is to propose a process for the manufacture of Flexible Terminal Connections by Helium protected Arc Welding (HePAW) process, using inert gas shielding of pure Helium.
Another object of the present invention is to propose a process for manufacture of Flexible Terminal Connections wherein the process is simple and does not require any skilled operators.
Still another object of the present invention is to propose a process for manufacture of FTC wherein the process is based on a technology which is readily available within the country.
Further object of the present invention is to propose a process for manufacture of FTC, which is cheaper and requires cheaper infrastructure.
Still further object of the present inventors is to propose a process for manufacture of FTC, which can be carried out at any open place and in any position of welding.
STATEMENT OF INVENTION
According to this invention there is provided a process for manufacture of flexible terminal connections for use in large turbogenerators comprising Helium protected Arc Welding of G-Cu-CrF35 casting (5) at either end of Electrolytic Tough Pitch grade copper flexible strips (6) having copper content of 99.9% and phosphorus content of 0.003%, using S-Cu-Sn filler metal electrode of diameter 3mm with flux and using inert gas shielding of pure helium at the flow rate of 10-15 1/min, and travel speed of 60-90mm/min wherein the said G-Cu-CrF35 casting (5), has copper content greater than 97% and chromium content in the range of 0.40 to 1.4%, the said grade copper flexible strips (6) are pre-heated to a temperature of 450 to 500°C.
DESCRIPTION OF FIGURES
The present invention will now be illustrated with accompanying drawings which are illustrative embodiment of the invention and are not intended to be taken restrictively to imply any limitation on the scope of the present invention. In the accompanying figures:-

Fig. 1: shows the general arrangement of Flexible terminal connections in large trubogenerators;
Fig. 2: shows flexible terminal connections prepared by Electron beam welding;
Fig. 3: shows flexible terminal connections prepared by Helium protected Arc Welding (HePAW);
DESCRIPTION OF INVENTION
According to this invention, flexible terminal connections for use in large turbogenerators are manufactured using Helium protected Arc Welding (HePAW). Base metals used are G-Cu-CrF35 casting and ETP Cu expansion strips. The said G-Cu-CrF35 casting has copper content higher than 97% and chromium content in the range of 0.40-1.40%. The said ETP Cu expansion strips have copper to the extent of 99.9% and phosphorus (P) to the extent of 0.003%. The joint has weld angle of 30 to 35° in casting and 10-15° in copper flexible. The filler metal electrode used is S-Cu-Sn electrode having dia of 3mm. The gas shielding is by using pure helium gas at a flow-rate of 10-15 1/min. The base material namely castings and copper flexibles are pre-heated to a temperature between metal before welding are pre-heated to a temperature in the range of 450° to 500°C. The Weld speed is 60-90mm/min.
Testing on Mock-up Piece Weld Joint;
All the weld joints made by Helium Protected Arc Welding (HePAW) Process were examined visually and subjected to destructive and non-

destructive examination to evaluate the integrity and quality of the same. For non-destructive testing, visual Examination under 5x magnification followed by Liquid Penetration Test on the welded joint (after surface conditioning by grinding) revealed no surface defects and discontinuities. The subsequent Radiographic Examination showed that weld joints were with acceptable level of porosity when all the weld procedure conditions were satisfied and welding was completed uninterrupted. Also from electrical point of view, mili-volt drop across the weld joint was assessed and the same was in order.
For Destructive Examination, the specimen were prepared by machining of the transverse cut section of the weld joint as per requirement of ASME Section IX standard. The results obtained from the tests showed that Ultimate Tensile Strength (UTS) values were more than that of the parent metal and hence acceptable. Further samples were tested for Bend Test on both root and face and all the samples demonstrated that there was no open discontinuity.

Type Tests:
Natural Frequency Test was carried out to ensure that none of the natural frequency was close to 50Hz or 100 Hz.
Vibration Fatigue Test
A very rigorous test for ascertaining the quality of the weld joint was carried out. This was done for a period corresponding to 100 million cycles with an amplitude level of 1000 microns for both 50 Hz and 100 Hz frequencies. No cracks or abnormality was revealed after this test which shows the excellent quality and reliability of the joint.
FTC thus manufactures were assembled on a 500 MW generator during works testing and withstood successfully 109% of the rated current without any adverse effect. These FTC also successfully sustained the sudden short circuit test thereby confirming both electrical and mechanical adequacy of the weld joints.
It is to be noted that present invention is susceptible to modifications changes and adaptations by those skilled in the art. Such, variant embodiments incorporating the concepts and features of the present invention, are intended to be within the scope of the present invention, which is further set forth under the following claims:

WE CLAIM:
1. A process for manufacture of flexible terminal connections for use in large turbogenerators comprising Helium protected Arc Welding of G-Cu-CrF35 casting (5) at either end of Electrolytic Tough Pitch grade copper flexible strips (6) having copper content of 99.9% and phosphorus content of 0.003%, using S-Cu-Sn filler metal electrode of diameter 3mm with flux and using inert gas shielding of pure helium at the flow rate of 10-15 1/min, and travel speed of 60-90mm/min wherein the said G-Cu-CrF35 casting (5), has copper content greater than 97% and chromium content in the range of 0.40 to 1.4%, the said grade copper flexible strips (6) are pre-heated to a temperature of 450 to 500°C.
2. A process as claimed in claim 1, wherein weld angle for said casting is 30 to 35°.
3. A process as claimed in claim 1, wherein weld angle for said copper flexible strips is 10 to 15°.
4. A process for manufacture of flexible terminal connections for use in large turbogenerators as substantially herein described and illustrated.